fix(devices): cache platform device names, dedupe placeholder ids, purge collided EQ entries

Dart half of bt-device-identity. EstadoEcualizador now caches each
device's platform-reported name in memory so the settings screen
shows the device's own Bluetooth name instead of its raw id when no
custom rename exists, and skips auto-creating preset entries for the
composite-placeholder sentinel. Enabling multi-device EQ triggers the
Bluetooth permission request through the new channel contract. A
flag-guarded one-time migration purges only entries keyed by the
exact literal placeholder id from the three per-device preference
maps, since those collided entries cannot be attributed to a device.

Work unit 2/2 of bt-device-identity (Dart state + migration).

lib/servicios/servicio_dispositivo_audio.dart

@@ -21,6 +21,12 @@ abstract class ServicioDispositivoAudio {
   /// (method channel round-trip). Returns the cached value if already known.
   Future<DispositivoAudio> obtenerDispositivoActual();
 
+  /// Requests the `BLUETOOTH_CONNECT` runtime permission (API 31+) at the
+  /// point the device-management UI is opened (bt-device-identity ADR-1).
+  /// Returns true when granted or not required (SDK < 31, iOS); false when
+  /// denied.
+  Future<bool> solicitarPermisoBluetooth();
+
   /// Cancels the device-change subscription and releases resources.
   Future<void> dispose();
 }
@@ -82,6 +88,14 @@ class ServicioDispositivoAudioReal extends ServicioDispositivoAudio {
     return device;
   }
 
+  @override
+  Future<bool> solicitarPermisoBluetooth() async {
+    final granted = await _methodChannel.invokeMethod<bool>(
+      'requestBluetoothConnect',
+    );
+    return granted ?? false;
+  }
+
   @override
   Future<void> dispose() async {
     await _eventSub?.cancel();